Evaluation of the performance of briquettes as potential feed material to the rotary kiln electric furnace at Barro Alto, Brazil

Abstract

The Barro Alto nickel laterite deposit, characterized by three main ore types (West, East, and Plain), employs a processing plant equipped with two 185 m rotary kilns and six-in-line rectangular electric arc furnaces. This study investigated the impact of briquetting of the ferronickel ore, premised on the minimization of fine particles in the rotary kiln, as well as improving charge permeability in the furnace and overall process safety. ISO standard tests for iron ore pellets were used to evaluate the physical and metallurgical properties of briquettes prepared under different conditions.

Briquetting tests were conducted on screened nickel laterite ore (−6.3 mm and −12.5 mm) at moisture contents of 16% and 24% and cured under closed and open conditions. Results revealed that, at a briquette ore top size of −6.3 mm and 16% moisture content, the average green briquette compressive strength was 559 N and the drop number was 19.1; however, an increase in moisture content to 24% had a detrimental effect on green both briquette compressive strength and yield. At a briquette ore top size of -6.3 mm and a 16% moisture content, an average green briquette yield of 85.2% was reported .Increasing the moisture content proved to have a detrimental effect on the green briquette yield, with an average green briquette yield of 67.7% reported at a moisture content of 24%.

Size degradation resulting from reduction indicated that the degree of disintegration of the briquettes (RDI−0.5mm) increased with an increase in moisture content for briquette ore top sizes of −6.3 mm and −12.5 mm. For the −6.3 mm briquette ore top size, RDI−0.5mm increased from 1.76% at 16% moisture to 3.33% at 24% moisture content.

The ISO 8371 test method was used to evaluate decrepitation of briquettes on entry into the rotary kiln. Results indicated that moisture significantly influenced decrepitation compared with curing conditions. For the −6.3 mm briquette ore top size, the average decrepitation index was 0.33% at 16% moisture content; this increased to 0.61% at 24% moisture content. For the −12.5 mm briquette ore top size, the decrepitation index was 2.34% at 16% moisture content and 1.96% at 24% moisture content.

The degree of reduction was evaluated using the ISO 4695 test method. At 16% moisture content, the average iron reduction was 33.4% under open and closed curing conditions for −6.3 mm briquette ore top size and 39.9% for −12.5 mm briquette ore top size; at 24% moisture content, the average iron reduction was 27.6% for −6.3 mm briquette ore top size and 36.9% for −12.5 mm briquette ore top size.

A tumble index of 95.5% was reported at 16% moisture content under open curing conditions at briquette ore top size of -6.3 mm. At briquette ore top size of -12.5 mm the tumble index dropped significantly to 40.3%. The results indicated that the tumble index is significantly affected by the particle size distribution, a similar trend was also observed under closed curing conditions. At 16% moisture content under closed curing conditions a tumble index of 91.4% was reported at briquette ore top size of -6.3 mm and 40.7% at briquette ore top size of -12.5 mm

Based on the findings of the test programme, optimal structural integrity and process stability can be achieved using briquettes produced at an ore top size of −6.3 mm, with a moisture content of 16%, and cured under open conditions.